1. Field of Invention
The present invention relates to a side light type light source device for use with a liquid-crystal display or the like, and more particularly to a side light type surface light source device that employs a light guide plate having emitting directivity.
2. Related Art
Side light type surface light source devices have a thin structure and have previously have been applied to backlighting of liquid-crystal display panels and the like.
A side light type surface light source device is equipped with a light guide plate, and a rod-shaped light source (primary light source) such as a cold-cathode tube arranged along an edge of the light guide plate. Light emitted by the primary light source is introduced through an edge of the light guide plate into the light guide plate. The light introduced into the light guide plate is deflected and output from one face of the light guide plate to illuminate, for example, a liquid-crystal panel.
Light guide plates employed in side light type surface light source devices are classified basically into two types by shape. One of these types of light guide plate has a substantially uniform thickness, while the other type of light guide plate has a thickness that has a tendency to decrease gradually from one edge. It is known that the latter type can emit light generally more efficiently than the former type.
FIG. 6 is an exploded perspective view showing a general configuration of a side light type surface light source device of the latter type. With reference to FIG. 6, a side light type surface light source device 1 is provided with a light scattering guide plate 2 (a light guide plate made of a light scattering and guiding material), and a primary light source 3 disposed at the side of the light scattering guide plate 2. The light scattering guide plate 2 is in a lamination arrangement with a reflection sheet 4 and a light control member comprised by a prism sheet 5. The primary light source 3 includes for example a cold-cathode tube (fluorescent lamp) 6 and a reflection member (reflector) 7 with a semicircular cross-section facing a portion of the circumference of the cold-cathode tube (fluorescent lamp) 6. Light radiating from the cold-cathode tube (fluorescent lamp) 6 impinges onto the edge surface (hereinafter referred to as the xe2x80x9cincident surfacexe2x80x9d) of the light scattering guide plate 2 from the open portion of the reflector 7.
The reflection sheet 4 is a sheet-shaped regular reflection member formed of gold foil or the like, or a sheet-shaped irregular reflection member formed of white PET film or the like.
The light scattering guide plate 2 has a wedge-shaped cross-section. The material of the light scattering guide plate 2 is obtained by uniformly distributing light-permeable fine particles, for example, in a polymethylmethacrylate (PMMA) matrix having a different refractive index from that of the light-permeable fine particles.
FIGS. 7A and 7B are cross-sectional views along line Axe2x80x94A of FIG. 6, and also show the light path that explains the behavior of the light. With reference to FIG. 7A, light L from the primary light source 3 is guided into the light scattering guide plate 2 from the incident surface T. The light-permeable fine particles distributed in the light scattering guide plate 2 scatter the light L. When an irregular reflection member is employed as for the reflection sheet 4, a portion of the light is also scattered by the reflection sheet 4.
The light L is thus subjected to this scattering and nears the tip of the light scattering guide plate 2 as it is reflected between the surface on the reflection sheet 4 side (hereinafter referred to as the xe2x80x9csloping surfacexe2x80x9d) and the surface on the prism sheet 5 side (hereinafter referred to as the xe2x80x9cemitting surfacexe2x80x9d).
In this propagation process the angle of incidence of the light L relative to the emitting surface is reduced little by little each time the light L is reflected by the sloping surface. Components satisfying the condition of being at or below the critical angle relative to the emitting surface exit from the emitting surface. Based on the above-described scattering, light L1 exiting from the emitting surface, as shown in FIG. 7B, has scattering light properties. However, light L1 does not propagate in perfectly random directions, but has a preferential propagation direction.
As shown in FIG. 7B, this preferential propagation direction is inclined toward the tip of the wedge relative to the emitting surface. In other words, light L1 emitted from the light guide plate has directivity, whereby emitting directivity is imparted to the side light type surface light source device 1.
Prism sheet 5 is arranged to correct this directivity. Prism sheet 5 is formed of light-permeable sheet material such as polycarbonate, with the prism surface facing the light scattering guide plate 2 (i.e. inwardly). The prism surface is comprised of numerous triangular-cross-section projections extending substantially parallel to the incident surface T of the light scattering guide plate 2. The sloping surfaces of these triangular projections correct the principal emission direction of the light. L1 (i.e. preferential propagation direction) to the front of the emission surface.
The prism sheet 5 may be a double-sided prism sheet. A double-sided prism sheet is one that has another prism surface on the side opposite to the light scattering guide plate 2 side (outside surface). The grooves on the outside prism surface are substantially at right-angles to the grooves on the inside prism surface. A surface light source device 1 that employs these prism sheets emits light to the front with good efficiency, compared with side light type surface light source devices that employ a light guide plate of substantially uniform thickness.
Light guide plates having emitting directivity include wedge-shaped or nearly wedge-shaped light guide plates comprised of a transparent or semi-transparent material, and flat-plate-shaped light guide plates having a scattering film or the like on the emitting surface, or on the reverse surface, or on both surfaces. A side light type surface light source device that employs a plate such as these also is able to emit light to the front with good efficiency.
The present inventors observed that if this type of side light type surface light source device is placed in a high-temperature environment for a long time, various patterns appear on the prism sheet 5 (that is, on the illuminant surface of the side light type surface light source device), as shown in FIG. 8. This phenomenon can be explained as follows.
When a prism sheet 5 is exposed to a high-temperature environment for a long time, the prism sheet 5 tends to exhibit local adhesion to the emitting surface of the light scattering guide plate 2. In regions where the prism sheet 5 adheres to the emitting surface, the layer of air between the prism sheet 5 and the emitting surface is lost. This results in disturbances in the thickness distribution of the air layer, thereby providing the pattern appearance.
Island-shaped patterns C as shown in FIG. 8 appear when the prism sheet 5 adheres to areas of the emitting surface of the light scattering guide plate 2. Spot adhesions of the prism sheet 5 to the emitting surface of the light scattering guide plate 2 gives rise to a pattern of spots D. These patterns degrade the quality of the surface light source device""s light. Further, when applied to a liquid-crystal display backlighting arrangement, it degrades the quality of the display screen.
The aim of the present invention is to resolve the above problems of the prior art. The object of the present invention is to provide a side light type surface light source device that can prevent adhesion of a light control member such as a prism sheet to the emitting surface, thereby avoiding reduction in illumination light quality.
In accordance with this invention, means to prevent adhesion between the emitting surface of the light guide plate and a plate-shaped control member used in the side light type surface light source device is applied to at least one of the light guide plate emitting surface and the light control member.
The adhesion prevention means applied to the emitting surface is embodied as an emitting surface having a roughness that does not disorder the emitted light. The emitting surface roughness may be provided by roughening the emitting surface itself or by adhering numerous fine particles to the emitting surface.
The adhesion prevention means applied to the light control member is embodied as providing the light control member with projections having various heights, thereby effectively avoiding the light control member from adhesion.
The roughness imparted to the emitting surface preferably within arithmetic average roughness Ra range from 0.02 to 0.25 xcexcm. Such a roughness may be applied by roughening the emitting surface itself or by adhering numerous fine particles to the emitting surface.
In accordance with this invention, the roughness of the emitting surface holds diffusion of emitted light to within a range that is small enough for practical purposes, maintaining the directivity of the emitted light. Providing such a roughness on the emitting surface effectively prevents adhesion by lowering the affinity between the light control member and the emitting surface.
A light control member with projections of various heights causes the space between each projection and the emitting surface to vary accordingly. The result is that affinity between light control member and emitting surface is lowered, effectively preventing adhesion. This technique does not reduce emitted light directivity.
The invention will now be described in further detail with reference to the accompanying drawings.